Multiplex optical communication system

ABSTRACT

A multiplex optical communication system transmitting a multiplex optical signal between terminal equipments each including converters producing optical signals combined to the multiplex optical signal, through repeater equipment including an optical amplifier, under controlling the amplifier so that when a converter is added or removed, a time constant of the amplifier is equal to a time constant of the added or removed converter, or the amplifier produces output under constant output level control before adding or removing the converter and after the amplifier produces final output and constant gain control during the added or removed converter increases or decreases output. The time constant control is performed to the amplifier by making the amplifier repeat start and stop of increasing or decreasing output step by step in accordance with prescribed objective values corresponding to half way output of the optical amplifier.

CROSS-REFERENCES TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 09/090,823, filedJun. 3, 1998, which is a division of U.S. application Ser. No.08/587,390, filed Jan. 17, 1996, now U.S. Pat. No. 5,805,322.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a multiplex optical communicationsystem for transmitting multiplex optical signals under a signaltransmission technology of Wavelength Division Multiplexing (WDM) orOptical Time Division Multiplexing (OTDM).

In particular, the present invention relates to optical amplifiers foramplifying power of the multiplex optical signals transmitted throughthe system.

FIG. 1A is a block diagram for illustrating the multiplex opticalcommunication system of the related art, operating under the WDM or OTDMtechnology. The multiplex optical communication system is composed ofoptical signal terminating equipment (TERM EQUIP) (1 and 1′) placed atboth terminals of the OPT-TRANS LINE 3, for transmitting and receivingthe multiplex optical signals, an optical transmission line (OPT-TRANSLINE) (3) made of an optical fiber depicted by a thick line, fortransmitting the multiplex optical signal, and optical amplifierrepeater equipment (OAMP REP EQUIP) (2) placed along the OPT-TRANS LINE3, for amplifying and repeating the multiplex optical signalstransmitting between the TERM EQUIP 1 and 1′.

The TERM EQUIP 1 includes a transmitting unit (TX-UNIT) (1001) fortransmitting a multiplex optical signal to the TERM EQUIP 1′ and areceiving unit (RX-UNIT) (1002) for receiving a multiplex optical signaltransmitted from TERM EQUIP 1′. The same as TERM EQUIP 1, the TERM EQUIP1′ includes TX-UNIT 1001′ and RX-UNIT 1002′. The TERM EQUIP 1 and TERMEQUIP 1′ have the same constitution and function, so that the TERM EQUIP1 will be representatively described hereinafter. The OAMP REP EQUIP 2includes two repeater optical amplifiers (REP OPT-AMPs) (8 and 8′). TheREP OPT-AMP 8 is to amplify the multiplex optical signal transmittedfrom the TX-UNIT 1001 in the TERM EQUIP 1, for repeating the multiplexoptical signal to RX-UNIT 1002′ in the TERM EQUIP 1′, and the REPOPT-AMP8′ is to amplify the multiplex optical signal transmitted from theTX-UNIT 1001′ in the TERM EQUIP 1′, for repeating the multiplex opticalsignal to RX-UNIT 1002 in TERM EQUIP 1. When a distance between TERMEQUIP 1 and 1′ is long, a plurality of the OAMP REP EQUIP 2 are placed.However, one OAMP REP EQUIP 2 is representatively depicted in FIG. 1A.

FIG. 1B shows a block diagram of the TX-UNIT 1001 of the related art.The TX-UNIT 1001 consists of electro-optical signal converter (ELEC-OPTCONV) (4) connected with electrical signal channel lines (ELEC-SIGCHANNEL LINEs) (9) through which a plurality of electrical signalsformed to channels are sent to the ELEC-OPT CONV 4, an optical signalcombiner (OPT-SIG COMB) (5) connected with the ELEC-OPT CONV 4 throughoptical fibers depicted by thick lines, and a transmitting unit opticalamplifier (TX-UNIT OPT-AMP)(6) connected with the OPT-SIG COMB 5 throughan optical fiber depicted by a thick line. The ELEC-OPT CONV 4 is forconverting the electrical signals to optical signals at every channel.The ELEC-OPT CONV 4 consists of converters 4-1, 4-2, - - - and 4-n incorrespondence with the ELEC-SIG CHANNEL LINEs 9. When the electricalsignals are fed to the ELEC-OPT CONV 4 through the ELEC-SIG CHANNELLINEs 9, the converters 4-1, 4-2, - - - and 4-n convert the electricalsignals to optical signals and send the optical signals to the OPT-SIGCOMB 5, respectively. The OPT-SIG COMB 5 is for combining the opticalsignals sent from the ELEC-OPT CONV 4, adopting the WDM technology orthe OTDM technology, so as to produce a multiplex optical signal. TheTX-UNITOPT-AMP 6 is for amplifying the power of the multiplex opticalsignal sent from the OPT-SIG COMB 5. The amplified multiplex opticalsignal is sent out from the TX-UNIT 1001 to the REP OPT-AMP 8 in OAMPREP EQUIP 2 through the OPT-TRANS LINE 3.

FIG. 1C shows a block diagram of the RX-UNIT 1002. The RX-UNIT 1002consists of an optical signal branching unit (OPT-SIGBRANCH) (5′) andoptical-electro signal converters (OPT-ELEC CONVs) (4′). The OPT-SIGBRANCH 5′ is connected with the OPT-TRANS LINE 3 depicted by a thickline, for optically demultiplexing the received multiplex optical signalto a plurality of received optical signals which are called “receiveddemultiplexed optical signals” hereinafter. The received demultiplexedoptical signals produced at the OPT-SIG BRANCH 5′ are sent to theOPT-ELEC CONV 4′ through optical fibers depicted by thick lines. TheOPT-ELEC CONV 4′ consists of converters 4′-1, 4′-2, - - - , 4′-n atwhich the received demultiplexed optical signals are converted toreceived electrical signals and sent out from RX-UNIT 1002 to theELEC-SIG CHANNEL LINEs 9, respectively.

In FIG. 1A, the OAMP REP EQUIP 2 includes two optical amplifiers (8 and8′) which will be called REP OPT-AMPs 8 and 8′ hereinafter. The REPOPT-AMP 8 and 8′ are for amplifying the power of multiplex opticalsignals received from TERM EQUIP 1 and 1′, respectively. By virtue ofthe REP OPT-AMPs 8 and 8′, power loss, caused by the OPT-TRANS LINE 3,of the multiplex optical signals transmitting between TERM EQUIP 1 and1′ are recovered. Therefore, when a length of the OPT-TRANS LINE 3between TERM EQUIP 1 and 1′ is long, a plurality of the OAMP REP EQUIP 2are placed along the OPT-TRANS LINE 3, and the number of the OAMP REPEQUIP 2 is determined by considering both the power loss due to theOPT-TRANS LINE 3 and the power amplification factors of REP OPT-AMPs 8and 8′ in OAMP REP EQUIP 2, so that the multiplex optical signals can betransmitted between the TERM EQUIP 1 and 1′ in high fidelity and a highsignal to noise ratio (SNR).

Generally, there are two kinds of optical amplifiers, a semiconductoramplifier and an optical fiber amplifier. The both kinds of opticalamplifiers can be applied to the TX-UNIT OPT-AMP 6 in FIG. 1B and theREP OPT-AMPs 8 and 8′ in FIG. 1A. For example, in case the TX-UNITOPT-AMP 6 is the semiconductor amplifier, the multiplex optical signalfed to the TX-UNIT OPT-AMP 6 is amplified by a semiconductor deviceoperating under DC supply current, and in case the TX-UNIT OPT-AMP 6 isthe optical fiber amplifier, the multiplex optical signal fed to theTX-UNIT OPT-AMP 6 is amplified in an optically amplifying technologyusing an induced emission.

Recently, the optical fiber amplifier is used to the TX-UNIT OPT-AMP 6and the REP OPT-AMP 8 mostly. Because, the optical fiber amplifier hasfeatures such as a low Noise Figure, a little non-linearity inamplification, a low connection loss with the OPT-TRANSLINE 3, highcapability of a power amplification and a high stability against atemperature change. The optical fiber amplifier is composed of a rareearth metal-doped optical fiber such as Erbium (Er)-doped optical fiberand a pump light source such as a semiconductor laser.

In the multiplex optical communication system of the related art, theoutput power of the multiplex optical signal from the TX-UNITOPT AMP 6or the REP OPT-AMP 8 is controlled so as to be always constant in levelunder constant output level control performed in the TX-UNIT OPT AMP 6and the REP OPT-AMP 8 respectively. In case of the REP OPT-AMP 8, byvirtue of the constant output level control, the OAMP REP EQUIP 2 can beplaced independently on a length of the OPT-TRANS LINE 3 connected withthe OAMPREP EQUIP 2. In other words, the power level of each sectionbetween REP OPT-AMPs or between TERM EQUIP and REP OPT-AMPs isindependent. The change of power level and different of OPT-TRANS LINE 3loss at one section don't affect the power level at next section.

If a multiplex optical signal includes “n” channels and the TX-UNITOPT-AMP 6 is required to produce at least output power “P_(o)” per achannel for obtaining an advisable SNR, the TX-UNIT OPT AMP 6 must bedesigned so as to produce output power of “P_(o)×n”. In other words, theTX-UNIT OPT AMP 6 initially produces the optical output under theconstant output level control so that the output power of the TX-UNITOPT AMP 6 corresponds to the number of the channels of a multiplexoptical signal to be initially amplified by the TX-UNIT OPT AMP 6.

From a viewpoint of the operational flexibility of the multiplex opticalcommunication system, it is desirable that the channels of the multiplexoptical signal can be changed easily in response to trouble about thetransmission of the multiplex optical signal and up grade of trafficcapacity. For example, at first some channels which meet demand areused. When more traffic capacity are needed, other channels will becomeused. Usually, the multiplex optical communication system provides atleast one spare channel in place of a fallen channel. For example, whena module of a channel has trouble, another module of the spare channelis used instead of the troubled module. Such previous provision of thespare channel is effective for increasing the operational reliability ofthe multiplex optical communication system. However, when the sparechannel is used, there has been a problem of the output power in themultiplex optical communication system of the related art.

In the multiplex optical communication system of the related art, theconstant output level control is performed to optical amplifier so as tokeep the total output of the multiplex optical signal constant. As aresult, when the number of the channels decreases by removing a CONVwhich will be called “removed CONV” hereinafter, single output of eachchannel increases. On the contrary, when the number of the channelsincreases by adding a CONV which will be called “added CONV”hereinafter, the single output of each channel decreases.

When output power of a channel of the multiplex optical signal changesthus, a problem due to a non-linear effect occurs on the optical fiberof the OPT-TRANS LINE 3. That is, when the power of a channel exceeds aspecific level, a waveform of each channel is distorted by thenon-linear effect on the optical fiber. The non-linear effect isgenerally called a self phase modulation effect. Meanwhile, incontradiction to the above, the power of the optical signal is requiredto be larger than a specific level for maintaining a required SNR at thereceiving unit such as the RX-UNIT 1002 or 1002′.

In the REP OPT-AMP 8, minimum input power and maximum output power arerequired for performing the reception and the transmission of themultiplex optical signal safely. When there are a plurality of the REPOPT-AMPs 8 in the multiplex optical communication system, these minimuminput power and maximum output power are determined by the reception andamplification ability of each REP OPT-AMP 8 and the number of the REPOPT-AMPs 8. In each REP OPT-AMP 8, a level difference between theminimum input power and the maximum output power is called atransmission and reception level difference. The REP OPT-AMP 8 isdesigned so that the transmission and reception level difference islarger than a signal loss caused by the OPT-TRANS LINE 3 lying betweenthe OAMP REP EQUIP 2. Furthermore, in the design of the REP OPT-AMP 8, amargin of output power of each channel is afforded to insure its leveldifference caused by increase or decrease of the number of the channelsin the multiplex optical signal. Because of allowing the margin thus, ashare of the transmission and reception level difference to the opticaltransmission loss is decreased. In other words, a distance between theOAMP REP EQUIP 2 is shortened. This results in increasing the number ofthe OAMP REP EQUIP 2 uneconomically.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to make the multiplexoptical communication system have a large operational flexibilityagainst the variation of the total number of the channels in themultiplex optical signal transmitted through the system.

Another object of the present invention is to increase the operationalfidelity of the multiplex optical communication system.

Still another object of the present invention is to improvecontradiction occurring in the system that increasing output power isrequired to maintain the required high SNR, however, the output powercannot be increase so high because the self phase modulation effectoccurs in the system.

Further another object of the present invention is to decrease costs forconstructing and maintaining the repeater equipment by decreasing thenumber of the repeater equipment.

The above objects are achieved by controlling the terminal equipment andthe repeater equipment of the system so that output variation occurringat the terminal equipment is made equal to the output variationoccurring at the repeater equipment. Wherein, the converter is providedin the terminal equipment for converting an electrical signal to anoptical signal. The multiplex optical signal is formed by combining theconverted optical signals produced at the converters in corresponding toa plurality of the electrical signals fed to the terminal equipment. Theoutput variation is caused by changing the number of operatingconverters due to adding or removing a converter to or from theoperating converters.

In order to make the optical output variations equal to each other,either of two ways is operated to the optical amplifier of the repeaterequipment while the added converter is increasing output or the removedconverter is decreasing output. One way is “to change the output powerof the optical amplifier under the same time-constant as the outputchange of the converter” which is called “time constant control”.Another way is “constant gain control” which is control for keeping gainof the optical amplifier constant.

First of all, “time constant control” will be explained.

The time constant at changing the target of output level of the opticalamplifier is set as same as the time constant of the added converter orthe removed converter, then the output power of each converter is keptconstant even when converter is added or removed newly. Before adding orremoving a converter, the terminal and the repeater equipment produceoutput under “constant output level control” which is for keeping outputof the multiplex optical signal produced from the terminal and therepeater equipment, constant. When a converter is added or removed, theoptical amplifier in the repeater equipment changes the target of theprescribed value under the same time constant as the output change ofthe converter. In order to perform changing the target of output leveland announce the prescribed value, a monitor controller is provided tothe terminal equipment for generating an optical output control signalto be sent to the optical amplifier in the repeater equipment.

When a converter is added or removed, the monitor controller monitorsthe output of the converters and prohibits that an added or removedconverter starts to increase or decrease the output and sends theoptical output control signal to the optical amplifier in the repeaterequipment. After that, the monitor controller permits that the added orthe removed converter starts to raise or decrease the output power. Theoptical amplifier receives the optical output control signal and startsto change the target of output level to the prescribed value which isinformation on the optical output control signal announced from themonitor controller.

The prescribed value is proportional to the number of the converters atthe terminal equipment.

Next “constant gain control” will be explained. Before adding orremoving the converter, the optical amplifier of the repeater equipmentproduces output under “constant output level control” which is forkeeping output of the multiplex optical signal produced from theterminal and the repeater equipment, constant.

When a converter is added or removed, the monitor controller whichmonitors the number and the output power of converters, prohibits thatthe added or the removed converter starts increasing or decreasingoutput, and sends the optical output control signal to the opticalamplifier. After that, the monitor controller permits the added or theremoved converter starts increasing or decreasing the output power. Uponreceiving the optical output control signal, the optical amplifierchanges “constant output level control” to “constant gain control”.

After the added or the removed converter finishes increasing ordecreasing the output power, the control of the optical amplifierreturns to the “constant output level control” from “constant gaincontrol”. However, at this time, a constant output level is differentfrom the previous one, because it is a prescribed value determined bythe number of the converters. This is announced to the optical amplifierby the optical output control signal.

This switching from “constant gain control” to “constant output levelcontrol” is performed by the optical output control signal orautomatically, by no signal, after time, which is enough for the addedor the removed converter to finish increasing or decreasing the outputpower, passed.

When the optical amplifier produces output under the time constantcontrol, a delay occurs between the output from the added or removedconverter and the output from the optical amplifier. The delay isshortened by using step-by-step time constant control which is performedby providing half way objective values in the monitor controller so thatthe added or the removed converter produces output step by step underthe previously determined rising or falling time constant. Every timethe added or removed converter starts and stops increasing or decreasingoutput, the optical output control signal including the startinformation is sent from the monitor controller to the opticalamplifier. As a result, the optical amplifier repeats starting andstopping the increase or decrease of the output corresponding to theobjective values. By virtue of the set-by-step control, the error due tothe delay is reduced on an average.

The optical output control signal is transmitted between the terminalequipment and the repeater equipment through an optical transmissionline connecting them.

The improvement described above is performed to the multiplex opticalcommunication system operating under Wavelength Division Multiplexing(WDM) or Time Division Multiplexing (OTDM).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of a multiplex optical communication systemof the related art;

FIG. 1B is a block diagram of a terminal equipment 1001 in the multiplexoptical communication system of the related art, in a transmitting mode;

FIG. 1C is a block diagram of another terminal equipment 1002 in themultiplex optical communication system of the related art, in areceiving mode;

FIG. 2 is a block diagram of a multiplex optical communication system ofthe first embodiment of the present invention;

FIG. 3 is a block diagram of a transmitting unit in the multiplexoptical communication system of the first embodiment;

FIG. 4 is a block diagram of an optical amplifier repeater equipment inthe multiplex optical communication system of the first embodiment,illustrating a connected state with the transmitting unit;

FIG. 5 is a flow chart for illustrating operation steps in case of thefirst embodiment;

FIG. 6 is a graph for illustrating an increasing state of optical outputfrom a transmitting unit optical amplifier or a repeater opticalamplifier, in case of the first embodiment;

FIG. 7 is a graph for illustrating an increasing state of optical outputfrom a transmitting unit optical amplifier or a repeater opticalamplifier, in case of a second embodiment of the present invention;

FIG. 8 is a block diagram of an electro-optical signal converter forillustrating the first embodiment;

FIG. 9 is a block diagram of the transmitting unit optical amplifier orthe repeater optical amplifier, in case of the first embodiment;

FIG. 10 is a block diagram of a monitor controller of the transmittingunit, in case of the first embodiment;

FIG. 11 is a block diagram of the transmitting unit optical amplifier orthe repeater optical amplifier, in case of the first embodiment;

FIG. 12 is a flow chart for illustrating operation steps in case of athird embodiment of the present invention;

FIG. 13 is a block diagram of the transmitting unit optical amplifier orthe repeater optical amplifier, in case of the third embodiment;

FIG. 14 is a block diagram of the transmitting unit optical amplifier orthe repeater optical amplifier, in case of the third embodiment;

FIG. 15 is a block diagram of the transmitting unit in case of a fourthembodiment of the present invention;

FIG. 16 is a block diagram of the transmitting unit optical amplifier orthe repeater optical amplifier, in case of the fourth embodiment;

FIG. 17A is a block diagram of the multiplex optical communicationsystem of a fifth embodiment of the present invention;

FIG. 17B is a block diagram of a transmitting unit in the multiplexoptical communication system of the fifth embodiment;

FIG. 17C is a block diagram of the optical amplifier repeater equipmentin the multiplex optical communication system of the fifth embodiment,illustrating a connected state with the transmitting unit; and

FIG. 18 is wave forms illustrating a multiplex optical signal in case ofOptical Time Division Multiplexing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a block diagram for illustrating a multiplex opticalcommunication system of a first embodiment of the present invention. Inthe first embodiment, the multiplex optical communication systemtransmits signals or data under the OTDM. In FIG. 2, the multiplexoptical communication system principally consists of TERM EQUIP 100 and100′, OAMP REP EQUIP 200 and an OPT-TRANS LINE 3 depicted by a thickline. The TERM EQUIP 100 consists of a TX-UNIT 1011 and a RX-UNIT 1002,the TERM EQUIP 100′ consists of a TX-UNIT1011′ and an RX-UNIT 1002′ andthe OAMP REP EQUIP 200 includes REP OPT-AMPs 81 and 81′. In FIG. 2, thesame reference symbol as in FIG. 1A designates the same unit as in FIG.1A. There is a case where a plurality of OAMP REP EQUIP 200 are placedbetween the TERM EQUIP 100 and 100′ along the OPT-TRANS LINE 3. However,only one OAMP REP EQUIP 200 is representatively depicted in FIG. 2. Amultiplex optical signal transmitted from TX-UNIT 1011 (1011′) isamplified by the REP OPT-AMP 81 (81′) for repeating the multiplexoptical signal to the RX-UNIT 1002′ (1002). The present inventionrelates to optical amplifiers provided in a multiplex opticalcommunication system, so that the present invention relates to TX-UNITs1011 and 1011′ and REP OPT-AMPs 81 and 81′ as shown by blocks diagonallyshaded in FIG. 2. However, the TX-UNITs 1011 and 1011′, REP OPT-AMPs 81and 81′ have the same function and constitution respectively, so that inthe present invention, the TX-UNIT 1011 and REP OPT-AMP 81 will berepresentatively described in reference with FIGS. 3 and 4.

FIG. 3 is a block diagram for illustrating the TX-UNIT 1011 and FIG. 4is a block diagram for illustrating a relationship between the TX-UNIT1011 and the REP OPT-AMP 81. In FIGS. 3 and 4, the same reference symbolas in FIG. 1B designates the same unit as in FIG. 1B.

In FIG. 3, the TX-UNIT 1011 consists of an ELEC-OPT CONV 4, an OPT-SIGCOMB 5 optically connected with the ELEC-OPT CONV 4 through opticalfibers depicted by thick lines, a TX-UNIT OPT-AMP 6 optically connectedwith the OPT-SIG COMB 5 by an optical fiber depicted by a thick line anda monitor controller (MON CONT) (7) electrically connected with theELEC-OPT CONV 4, the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81,respectively. The ELEC-OPT CONV 4 consists of converter modules (CONVs)(4-1, 4-2, - - - and 4-n) each including an electro-optically convertingcircuit (CONV-CIRCUIT) (10), an optical output monitor (OPT-OUT MON)(11) and an optical output controller (OPT-OUT CONT) (12).

Electrical signals formed to channels are fed to the CONV-CIRCUITs inthe CONVs 4-1, 4-2, - - - and 4-n through ELEC-SIGCHANNEL LINEs 9-1,9-2, - - - and 9-n, respectively. In case of the WDM, the CONV-CIRCUITs10 produce optical signals having a different frequency or wavelengtheach other. The OPT-OUT MONs 11 monitor the optical output from theCONV-CIRCUITs 10 respectively, producing monitored optical outputsignals. The monitored optical output signals are sent to the MON CONT 7and to the OPT-OUT CONTs 12 in the CONVs 4-1, 4-2, - - - , 4-n.

The MON CONT 7 consists of a monitor processor (MON-PROC) (13), acontrol signal processor (CONT-PROC) (14) and a control signaltransmitter (CONT-SIG TX) (15). The monitored optical output signalssent from the OPT-OUT MONs 11 are collected at the MON-PROC 13,producing a collected monitored signal. The collected monitored signalis sent to the CONT-PROC 14 at which an optical output control signal isproduced and sent to both the OPT-OUT CONTs 12 and the CONT-SIG TX 15.The CONT-SIG TX 15 is for transmitting the optical output control signalto the TX-UNIT OPT-AMP 6 in TX-UNIT 1011 of the TERM EQUIP 100 and tothe REP OUT-AMP 81 in the OAMP REP EQUIP 200. There is a case where theTX-UNIT 1011 includes no TX-UNIT OPT-AMP 6. In this case, the opticaloutput control signal is sent only to the REP OPT-AMP 81. If there are aplurality of the OAMP REP EQUIP 200, the optical output control signalis sent to them through the REP OPT-AMP 81 in the OAMP REP EQUIP 200.

In each CONV, upon receiving the monitor output from the OPT-OUT MON 11and the optical output control signal from the CONT-PROC 14, the OPT-OUTCONT 12 performs stabilization and increase or decrease control of theoptical output from the CONV-CIRCUIT 10 by setting a rising or a fallingtime constant around the CONV-CIRCUIT 10.

In the MON CONT 7, when the MON-PROC 14 collects the optical outputmonitored signals from the OPT-OUT MONs 11 in the CONVs 4-1 to 4-n, theCONT-PROC 14 investigates how the optical signal is produced from theCONV-CIRCUIT 10 in each CONV. After the investigation, the CONT-PROC 14produces the optical output control signal to start or stop increasingor decreasing the optical output of the CONV-CIRCUIT 10 in a CONVrequired to be added or removed. The CONT-PROC 14 has another functionfor producing the start-stop control signal when a start signal “st” ora reset signal “rt” is given through an interface, not depicted in FIG.3, provided in the TERM EQUIP 100. The optical output control signalproduced at the CONT-PROC 14 is sent to the OPT-OUT CONT 12 in eachCONV, the TX-UNIT OPT-AMP 6 in the TX-UNIT 1011 and the REP OPT-AMP 81in the OAMP REP EQUIP 200 through the CONT-SIG TX 15.

In FIG. 3, if the number of the CONVs 4-1, 4-2, - - - and 4-n is “a”(n=a) and an optical output per a single channel, which will be called a“single channel optical output” hereinafter, from each CONV ishypothesized to be equally “1” in a steady state, and when a CONV4-(a+1) is newly added to the ELECT-OPT CONV 4, optical output of allchannels, which will be called “all channel optical output” hereinafter,of the TX-UNIT OPT-AMP 6 becomes as

a+1−exp(−t/τ 1).  (1)

In the expression (1), “t” is a lapse of time measured from time tostart operation of the CONV 4-(a+1) and “τ1” is a time constant requiredto make the CONV 4-(a+1) produce its single channel optical outputcompletely.

If a single channel optical output “p” is produced per each channel fromthe TX-UNIT OPT-AMP 6 or the REP OPT-AMP 8 and the all channel opticaloutput is increased from “p×a” to “p×(a+1)” after a time constant τ2 andwhen τ1 and τ2 are equal to τ1 (τ1=τ2=τ), the all channel optical outputfrom the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 8 can be expressed by:

p×a+{(p×(a+1)−(p×a))×{1−exp(−t/τ)}=p×a+p×{1−exp(−t/τ)}.  (2)

At this time, during the transitional state, the single channel opticaloutput from the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 is given as:

 p×a+p×{1−exp(−t/τ)}×{1−exp(−t/τ)}×{1/(a+1−exp(−t/τ)}=p.  (3)

The expression (3) signifies that when a CONV 4-(a+1) is added to theELEC-OPT CONV 4 and starts to operate, a single channel optical outputfrom the TX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) can be kept to “p”,by making the time constant τ2 equal to the time constant τ1. By virtueof changing output power of the TX-UNIT OPT-AMP 6 (and the REP OPT-AMP81) as controlling the time constant τ2 so as to be nearly equal to τ1,the problem of increasing the waveform distortion occurring in themultiplex optical signal due to the non-linear effect of the OPT-TRANSLINE 3 can be solved.

In opposition to the above, when one of the CONVs 4-1 to 4-a is removedby stopping operation, a single channel optical output from the TX-UNITOPT-AMP 6 (and the REP OPT-AMP 81) can be always kept to “p”, bycontrolling a decrease rate of the all channel optical output from theTX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) so that a time constant (τ4)required for decreasing the level of the all channel optical output fromthe TX-UNIT OPT-AMP 6 (and the REP OPT-AMP 81) to a value “p×(a−1)” isthe same as a time constant (τ3) required for extinguishing the singlechannel optical output of the removed CONV to a zero level. By virtue ofcontrolling thus, the problem of increasing the waveform distortionoccurring in the multiplex optical signal due to the non-linear effectof the OPT-TRANS LINE 3 can be solved.

The time constant such as τ1 or τ2 in the above description will becalled “rising time constant” and the time constant such as τ3 or τ4will be called “falling time constant” hereinafter.

In FIG. 3, the MON CONT 7 always monitors the optical output from theCONVs 4-1 to 4-n at the MON-PROC 13, and when a level of the opticaloutput from a CONV is varied, the variation is processed at the MON-PROC13 and the processed result is sent to the CONT-PROC 14 at which theoptical output control signal is produced for controlling the allchannel optical output from the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81in correspondence with the level variation of the optical output of theCONVs 4-1 to 4-n. The optical output control signal is transmitted fromthe CONT-SIG TX 15 to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81respectively.

The first embodiment of the present invention is based on controllingthe time constant τ2 so as to be equal to τ1 as described in referencewith the expression (3). The “control” will be called “time constantcontrol” and system on the “time constant control” will be called “timeconstant control system” hereinafter. FIG. 5 is a flow chart forillustrating the operation steps in case of the first embodiment. In thefirst embodiment, the ELECT-OPT CONV 4, the MON CONT 7 and the TX-UNITOPT-AMP 5 in the TX-UNIT 1011 of the TERM EQUIP 100 and the REP OPT-AMP81 in OAMP REP EQUIP 200 operate in accordance with steps A1 to A9 inFIG. 5. In the description of the steps A1 to A9, the added CONV will beused for the CONV 4-(a+1) and a removed CONV will be used for a CONVremoved from the ELEC-OPT CONV 4 hereinafter.

Whether CONV is added or removed is determined by a maintenance worker(step A1, CONV ADDED OR REMOVED?). In case a CONV is added, the addedinformation is sent to the MON CONT 7 by an installation monitor unitwhich will be described later, a higher rank apparatus not depicted inFIGS. 3 and 5, or the maintenance worker (step A2, SEND ADD-INF). Uponreceiving the added information, the CONT-PROC 14 in the MON CONT 7issues a command to raise optical output to the added CONV, and theCONT-SIG TX 15 in the MON CONT 7 issues a command to raise opticaloutput to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 respectively(step A3, RAISE OUTPUT OF ADD-CONV AND OPT-AMPs). The added CONVincreases the single channel optical output to a prescribed value by thecommand issued from the CONT-PROC 14 passing through the OPT-OUT CONT 12in the added CONV, and the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81increase the optical output at a rate determined by the time constant τ2which is previously set up in equal to the rising time constant τ1 ofthe added CONV (step A4, OPT-AMPs INCREASE OUTPUT WITH τ2 EQUAL TO τ1 OFADD-CONV). When the single channel optical output of the added CONVbecomes the prescribed value and the optical output from the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81 become respectively a value equal tothe product of the single channel optical output, the added CONV, theTX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 maintain the product valuerespectively. For example, the added CONV, the TX-UNIT OPT-AMP 6 and theREP OPT-AMP 81 maintain the product value “p×(a+1)” respectively in case“p” is the power of the single channel output and “a+1” is the totalnumber of the operating CONVs (step A5, OPT-AMPs OUTPUT p×(a+1)STEADILY).

In case a CONV is removed from the ELEC-OPT CONV 4 because of trouble ormaintenance, the remove information is sent to the MON CONT 7 from ahigher rank apparatus not depicted in FIGS. 3 and 5, or the maintenanceworker (step A6, SEND REMOV-INF). Upon receiving the removinginformation, the MON CONT 7 sends a command to decrease single channeloptical output to there moved CONV and a command to decrease the opticaloutput to the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 respectively(step A7, DECREASE OUTPUT OF REMOVE-CONV AND OPT-AMPs). Upon receivingthe decreasing command, the removed CONV decreases the single channeloptical output toward zero and the TX-UNIT OPT-AMP 6 and the REP OPT-AMP81 decrease the optical output at a rate determined by a time constant(τ4) which is previously set up in equal to the falling time constant(τ3) of there moved CONV (step A8, OPT-AMP DECREASE OUTPUT WITH τ4 EQUALTO τ3 OF REMOVE-CONV). Then, when the TX-UNITOPT-AMP 6 and the REPOPT-AMP 81 become respectively a value being the product of the singlechannel optical output, for example “p”, and the number, for example“a−1”, of operating CONVs, the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81maintain the product value “p×(a−1)” respectively and the removed CONVis completely stopped in operation (step A9, OPT-AMPs PRODUCEOUTPUTp×(a−1) STEADILY).

FIG. 6 is a graph for illustrating an increasing state of the opticaloutput from the TX-UNIT OPT-AMP 6 (or the REP OPT-AMP 81). In FIG. 6, ay-axis indicates the optical output of the TX-UNIT OPT-AMP 6 and anx-axis indicates time. When the TX-UNIT OPT-AMP 6 produces output “p”for a single optical signal and the operating number of the CONVs is“a”, the optical output of the TX-UNITOPT-AMP 6 becomes p×a finallywhich will be called P₁(p×a=P₁), and when a CONV is added to theELECT-OPT CONV 4 and the added CONV starts to operate at time t1, theoptical output of the TX-UNIT OPT-AMP 6 becomes p×(a+1) finally whichwill be called P₂(p×(a+1)=P₂). In FIG. 6, a solid line indicates anideal increasing state of the optical output of the TX-UNIT OPT-AMP 6from P₁ to P₂. A rate of increasing is determined by the rising timeconstant τ2 of the TX-UNIT OPT-AMP 6 which is controlled so as to beequal to the rising time constant τ1 of the added CONV. By virtue ofmaking the rising time constant τ2 equal to the rising time constant τ1,the output power of the single optical signal from the TX-UNIT OPT-AMP 6can be avoided varying. This results in preventing the waveformdistortion due to the non-linearity effect of the OPT-TRANS LINE 3 andthe SNR degradation from occurring in the multiplex opticalcommunication system.

However, there is a case where a time delay occurs between the timerequired to raise the optical output of the added CONV and a timerequired to increase the optical output of the TX-UNIT OPT-AMP 6. Adotted line in FIG. 6 indicates an actual state of increasing theoptical output of the TX-UNIT OPT-AMP 6 from P1 to P2 in case the timedelay occurs. FIG. 6 shows that though the added CONV rises the outputpower during time t1 to t2, the TX-UNIT OPT-AMP 6 increases the opticaloutput during time t1+Δt to t2+Δt having a delay time Δt. That is,because of the delay time Δt, an error ΔP₁ of output power appearsbetween the actual state of power increasing shown by the dotted lineand the ideal state of power increasing shown by the solid line.Incidentally, the time interval between t1 and t2 is several ms toseveral second and Δt is several % to approximately 25% of the timeinterval.

The error ΔP₁ of output power can be decreased by improving the timeconstant control so that the output power of the added CONV rises in astep-by-step manner as shown in FIG. 7. FIG. 7 illustrates thestep-by-step manner for increasing the output power of the added CONVunder the improved time constant control. In FIG. 7, symbols P₁₁, P₁₂and P₁₃ indicate halfway output from the TX-UNIT OPT-AMP 6 to be reachedat halfway times in the time interval between t1 and t3. The added CONVproduces optical output having levels corresponding to halfway objectivevalues, a first objective value, a second objective value and a thirdobjective value, in correspondence with P₁₁, P₁₂ and P₁₃ produced at thehalfway times respectively.

Before the added CONV is added, the output power of the TX-UNIT OPT-AMP6 is P₁ steadily. When the added CONV is added and it starts to raiseoptical output at time t1, the TX-UNIT OPT-AMP 6 starts to increaseoutput power under the time constant control, at time t1′ delayed anamount of Δt from time t1. When the optical output of the added CONVincreases as depicted by a thick line in FIG. 7 and reaches a levelcorresponding to the first objective value, the output of the added CONVis stopped to be raised until the TX-UNIT OPT-AMP 6 produces the opticaloutput P₁₁. After the optical output of the TX-UNIT OPT-AMP 6 increasesas depicted by a dotted line and reaches P₁₁, the added CONV starts torise the optical output at time t1″ and rises the optical output untilthe optical output reaches a level corresponding to the second objectivevalue. After delaying, Δt from time t1″, the TX-UNITOPT-AMP 6 starts torise the optical output and the optical output increases until becomingP₁₂. These steps are repeated step by step, increasing the output powerof the TX-UNIT OPT-AMP 6 toward P₂.

When the output of the TX-UNIT OPT-AMP 6 reaches P₂ at time t3, thestep-by-step increasing operation is ended.

In FIG. 7, under the step-by-step manner, an error ΔP₂ of the outputpower of the TX-UNIT OPT-AMP 6 appears between the actual increasingstate shown by the dotted line and the ideal increasing state as shownby the solid line. However, since the error ΔP₂ appears intermittentlyas shown in FIG. 7, an average error obtained by summing individualerror ΔP₂ from t1 to t2 becomes small in comparison with an averageerror the error ΔP₁ in between time t1 and time t2+Δt in FIG. 6. Thedetails of the multiplex optical communication system operating underthe time constant control performed in the step-by-step manner will bedescribed later as a second embodiment of the present invention.

FIG. 8 is a block diagram for illustrating one of the CONVs 4-1 to 4-nin case of the first embodiment. The one of the CONVs 4-1 to 4-n will becalled simply “CONV” hereinafter. In FIG. 8, the same reference numeralas in FIG. 3 designates the same unit as in FIG. 3. The CONV principallyconsists of the CONV-CIRCUIT 10, the OPT-OUT MON 11 and the OPT-OUT CONT12. The CONV-CIRCUIT 10 consists of a semiconductor laser diode (LD)(21) for emitting laser light, an optical modulator (OPT-MOD) (22) forproducing a single channel optical signal by intentionally modulatinglaser light emitted from the LD 21, and a driving circuit (DRIVE) (3)for driving the OPT-MOD 22 upon receiving an electrical signal or datato be transmitted, through the ELEC-SIG CHANNEL LINE 9. The OPT-OUTMON11 consists of an operation monitoring circuit (OPT-MON CIRCUIT) (24)for monitoring the output power of the laser upon receiving a part oflaser light emitted from the LD 21, an indicator (25) for indicating amonitored result obtained by receiving a part of monitored output fromthe OPT-MON CIRCUIT 24 and an installation monitor unit (INSTALL-MON)(26) for monitoring that the CONV is installed in the ELECT-OPT CONV 4.The OPT-OUT CONT 12 consists of a low pass filter (LPF) (27), a biasgenerating circuit (BIASGEN) (28) for generating a bias voltage appliedto the LD 21 and a switch (SWITCH) (29) for switching a source voltagefor the BIASGEN 28. The rising and falling time constant can bepreviously varied and set for the CONV by adjusting the LPF 27.Terminals “a, b, c and d” are for connecting the CONV with the MON CONT7, terminal “e” is a power source terminal and terminal “f” is aterminal connected with the ELECT-SIG CHANNEL LINE 9 for receiving theelectrical signal or data to be transmitted.

When the SWITCH 29 is ON by the optical output control signal sent fromthe MON CONT 7 through terminal “d”, DC power is applied to the BIAS GEN28 through terminal “e” and a bias voltage generated at the BIAS GEN 28is supplied to the LD 21 through the LPF27. The optical output powerfrom the LD 21 is monitored by the OPT-MON CIRCUIT 24, so that the LD 21transmits optical power finally having the objective value. In thiscase, the optical power transmitted from the LD 21 is gradually raisedin accordance with the bias voltage gradually raised. In other words,when the LD 21 is started to transmit the optical power, the opticalpower rises gradually at a rate determined by a time constant (τ1)corresponding to a rising rate of the bias voltage generated at the BIASGEN 28.

The OPT-MON CIRCUIT 24 monitors the optical output from the LD 21. Whenan accident occurs in the CONV, the OPT-MONCIRCUIT 24 issues a commandto stop the operation of the CONV through terminal “c” and at the sametime, the OPT-MON CIRCUIT 24 controls the indicator 25 so as to make theINDICATOR 25 indicate an accident occurrence. The indicator 25 has afunction of indicating a state of “normally operating”, “varying opticaloutput”, “accident occurs” or “stopping operations”. When many CONVs areinstalled in a rack, the indicator 25 is very helpful to distinct theoperation state of the CONVs and very useful to perform quick exchangeof a CONV when an accident occurs in the CONV.

The INSTALL-MON 26 informs to the MON CONT 7 that the CONV is installedin the ELEC-OPT CONV 4 by using a jumper wire provided on a back-boardof the INSTALL-MON 26. When the CONV is installed in the ELEC-OPT CONV4, terminals “a” and “b” is electrically connected by the jumper wire.By virtue of connecting “a” and “b”, the installation of the CONV to theELEC-OPT CONV 4 is informed to the MON CONT 7.

FIG. 9 is a block diagram for illustrating the TX-UNIT OPT-AMP 6 shownin FIG. 3 or the REP OPT-AMP 81 shown in FIG. 3, used in the firstembodiment. The TX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 consists of anoptical amplifier unit (OPT-AMP UNIT) (31), an optical output monitoringunit (OUTPUT MON) (32), a gain controlling unit (GAIN CONT) (33), anoutput constantly control unit (OUTPUT CONT) (34), an output levelsetting unit (LEVEL SET) (35), a control unit (CONT UNIT) (36) and acontrol signal receiving unit (CONT-SIG RX) (37).

The OPT-AMP UNIT 31 is an optical amplifier composed of an Erbium-dopedfiber amplifier and a pump light semiconductor laser. The OUTPUT CONT 34controls the OPT-AMP UNIT 31 through the GAIN CONT 33 so that theoptical output of the OPT-AMP UNIT 31 becomes a prescribed constantlevel by making a comparison between a level of optical output detectedby the OUTPUT MON 32 and an output level set at the LEVEL SET 35.

Upon receiving the optical output control signal from the MON CONT 7(refer FIG. 3), the CONT-SIG RX 37 sends the optical output controlsignal to the CONT UNIT 36 and other REP OPT-AMPs 81. Upon receiving theoptical output control signal, the CONT UNIT 36 controls the LEVEL SET35 so that the LEVEL SET 35 produces a setting level of the opticaloutput from the OPT-AMP UNIT 31 at a gradually increasing or decreasingrate. When the CONV is added, the setting level is gradually increasedin accordance with the rising time constant of the added CONV, and whenthe CONV is removed, the setting level is gradually decreased inaccordance with the falling time constant of the removed CONV. Thesetting level produced at the LEVEL SET 35 is sent to the OPT-AMP UNIT31 through the OUTPUT CONT 34 and the GAIN CONT 33 in order to increaseor decrease the level of the optical output from the OPT-AMPUNIT 31. Theoptical output level of the OPT-AMP UNIT 31 is monitored by the OUTPUTMON 32 and the monitored output from the OUTPUT MON 32 is sent to theOUTPUT CONT 34. When the monitor output reaches a level corresponding tothe number of the CONVs (refer FIG. 3), the OUTPUT CONT 34 controls sothat the level is always kept constant. Wherein, the number is increasedwhen CONV is added and the number is decreased when CONV is removed. TheTX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 will be detailed later inreference with FIG. 11.

FIG. 10 is a block diagram for illustrating the MON CONT 7 (refer FIG.3) used in the first embodiment of the present invention. In FIG. 10,the same reference numeral as in FIG. 3 designates the same unit as inFIG. 3 and the same reference symbol as in FIG. 8 designates the samepart as in FIG. 8. As shown in FIGS. 3 or 10, the MON CONT 7 principallyconsists of the MON-PROC 13, the CONT-PROC 14 and the CONT-SIG TX 15. InFIG. 10, the MON-PROC 13 and the CONT-PROC 14 are shown in adot-dash-line box respectively and the CONT-SIG TX 15 is shown in asolid line box.

The MON-PROC 13 consists of installation detecting circuits (INST-DETCIRCUIT) (41-1, 41-2, - - - and 41-n) and monitor control circuits(MON-CONT CIRCUITs) (45-1, 45-2, - - - and 45-n) in corresponding to theCONVs 4-1, 4-2, - - - and 4-n respectively. The INST-DET CIRCUITs 41-1,41-2, - - - and 41-n are for detecting whether the CONVs 4-1, 4-2, - - -and 4-n are installed in the TX-UNIT 1011 respectively. Each of theINST-DET CIRCUITs 41-1, 41-2, - - - and 41-n consists of a resister(42), a standard voltage source (43) and a comparator (COMP) (44).

When one of the CONVs 4-1, 4-2, - - - and 4-n is not installed in theELECT-OPT CONV 4, a terminal “a” and “b” is not electrically connectedwith the jumper wire in the INSTALL-MON 26 shown in FIG. 8. Thereforepotential at the terminal “b” becomes higher than earth potential at theterminal “a”. Upon detecting a difference of the potential between theterminals “a” and “b”, the COMP 44 detects that the removed CONV is notinstalled in the ELEC-OPT CONV 4. When, for example, a CONV is newlyinstalled in the TX-UNIT 1011, terminal “a” and “b” are connected withthe jumper wire in the INSTALL-MON 26, so that potential at the terminal“b” becomes equal to the earth potential at the terminal “a”. Upondetecting the equal earth potential at the terminal “a” and “b”, theCOMP 44 detects that the CONV is installed in the ELEC-OPT CONV 4. Uponreceiving the potential difference between the terminals “a” and “b”from the COMP 44 and the command to stop the operation of the removedCONV or to start the operation of the added CONV through terminal “c”,the MON-CONT CIRCUIT 45-1 sends a stop operation signal to a latchcircuit 46-x or a start operation signal to a latch circuit 46-(a+1) inthe CONT-PROC 14, respectively. The latch circuits are provided in theCONT-PROC 14 which will be described below.

The CONT-PROC 14 consists of latch circuits (LATCHes) (46-1, 46-2, - - -and 46-n), a trigger generator (TRIG GEN) (47), a counter (COUNT) (48)and a control signal generator (CONT-SIG GEN) (49).

The COUNT 48 holds the total number of the operating CONVs and producesa count up signal and performs up-counting or down-counting inaccordance with the stop operation signal or the start operation signalsent from relevant one of the MON-CONTCIRCUITs 45-1, 4-2, - - - and45-n, and when the counting is completed, the COUNT 48 controls the TRIGGEN 47 so that the TRIGGEN 47 adds a trigger signal to the LATCHes 46-1,4-2, - - - and 46-n and to the CONT-SIG GEN 49. Upon receiving thetrigger signal, the relevant one of the LATCHes 46-1, 4-2, - - - and46-n latches the stop operation signal or the start operation signalsent from the relevant one of MON-CONT CIRCUITs 45-1, 45-2, - - - and45-n. For example, when the CONV 4-1 is installed and starts to operate,the start operation signal produced at the MON-CONT 45-1 is sent toterminal “d” through the LATCH 46-1 opened up by a trigger signal sentfrom the TRIG GEN 47. The CONT-SIG GEN 49 is also controlled by anoutput signal from the COUNT 48 and the trigger signal from the TRIG GEN47, so that the CONT-SIG GEN 49 produces the optical output controlsignal for starting or stopping increasing or decreasing output of theTX-UNIT OPT-AMP 6 and REP OPT-AMP 81. Upon receiving the optical outputcontrol signal from the CONT-SIG GEN 49, the CONT-SIG TX 15 transmitsthe optical output control signal to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81 respectively.

FIG. 11 is a block diagram for illustrating a detail of the TX-UNITOPT-AMP 6 or the REP OPT-AMP 81 used in the first embodiment of thepresent invention. In FIG. 11, the same reference numeral as in FIG. 9designates the same unit as in FIG. 9. As described in reference withFIG. 9, the TX-UNIT OPT-AMP 6 or the REP OPT-AMP 81 principally consistsof the OPT-AMP UNIT 31, the OUTPUTMON 32, the GAIN CONT 33, the OUTPUTCONT 34, the LEVEL SET 35, the CONT UNIT 36, the CONT-SIG RX 37 and acontrol signal regenerator (CONT-SIG REGENE) (58). The GAIN CONT 33consists of an optical combiner (51) and a semiconductor laser (52), theOUTPUT MON 32 consists of an optical branch (53) and a photo diode (54),the OUTPUT CONT 34 consists of a transistor amplifier (55) and anoperational amplifier (56) and the LEVEL SET 35 includes a switchingcircuit (SW) (57). The OPT-AMP UNIT 31 is composed of an Erbium-dopedoptical fiber. FIG. 11 shows the Erbium-doped optical fiber in a casewhere forward pumping is performed by a semiconductor laser (52).Instead of the forward pumping, backward pumping or both forward andbackward pumping can be used.

Moreover, an isolator, not depicted in FIG. 11, for interrupting a turnback light of the OPT-AMP UNIT 31 can be added to the OPT-AMP UNIT 31.Usually, a wavelength in a band of 1.48 μm or 0.98 μm can be used for anoptical signal and wavelength in a band of 1.55 μm can be used for apumping light emitted from the laser 52.

The operational amplifier 56 compares one of reference values “rf1,rf2, - - - or rfm” switched by the SW 57 with the optical outputdetected at the OUTPUT MON 32, producing comparison output. Uponreceiving the comparison output from the operational amplifier 56, thetransistor amplifier 55 controls the laser 52 so that the pumping lightemitted from the laser 52 makes the OPT-AMP UNIT 31 produce the opticaloutput at a constant level.

The optical output control signal transmitted from the CONT-SIG TX 15(refer FIG. 3) is sent to the CONT UNIT 36 through the CONT-SIG RX 37.The CONT UNIT 36 controls the SW 57 so that the SW 57 switches eitherone of the reference values. By virtue of the switched reference value,the optical output of the OPT-AMP UNIT 31 becomes a level correspondingto the total number of actually operating CONVs, obtained afterincreasing or decreasing the CONV.

The reference values “rf1, rf2, - - - or rfm” are minutely provided andswitched by the SW 57, for increasing or decreasing the optical outputof the OPT-AMP UNIT 31 finally in corresponding to the rising timeconstant of the added CONV or the falling time constant of the removedCONV, respectively. The SW 57 is controlled by the CONT UNIT 36 so thatthe switching is closely performed as time passes in the rising orfalling process of the output power of the added or removed CONV. Wedesign that the time constant of the OUTPUT CONT 34 equal to the risingtime constant of the added CONV or the falling time constant of theremoved CONV.

The step-by-step manner as shown in FIG. 7 is also performed under thetime constant control performed by the MON CONT 7. In this case, one ofthe MON-CONT CIRCUITs 45-1 to 45-n and the CONT-SIG GEN 49 send theoptical output control signal to the OPT-MON CIRCUIT 24 in the CONV, theTX-UNIT OPT-AMP 6 and the REP OPT-AMP 81, under condition of the risingor falling time constant, the halfway objective values and prescribedtimes to attain the halfway objective value.

Upon setting the halfway objective values, the prescribed times and therising or falling time constant at the CONV such as the CONV 4-1,4-2, - - - or 4-n and the OPT-AMP such as the TX-UNIT OPT-AMP 6 and theREP OPT-AMP 81, the optical output from the TX-UNIT OPT-AMP 6 and theREP OPT-AMP 81 is controlled so as to be increased or decreased in thestep-by-step manner.

A third embodiment of the present invention is based on changing acontrolling manner of the optical output from the optical amplifier usedin the TX-UNIT OPT-AMP 6 and the REP OPT-AMP 81. System for changing thecontrol manner thus will be called “control manner change system”hereinafter. By virtue of the control manner change system, the opticalamplifier is controlled as follows:

usually the optical amplifier is controlled so as to produce opticaloutput at a constant level under the constant output level control, asdone in the prior art;

when a CONV is added to or removed from the ELEC-OPT CONV 4, theconstant output level control is changed to constant gain control, sothat the optical amplifier is controlled so as to produce the opticaloutput under the constant gain control; and

when the optical output of the added CONV is completely raised or theoptical output of the removed CONV is completely decreased, the constantgain control is changed back to the usual constant output level control,so that the optical amplifier is controlled so as to produce the opticaloutput under the constant output level control.

FIG. 12 is a flowchart for illustrating operation steps of the controlmanner change system used in the third embodiment. The OPT-AMP usuallyoperates under the constant output level control system. Upondetermining whether the CONV is added to or removed from the ELEC-OPTCONV 4 (step B1, CONV ADD or REMOVE?), the OPT-OUT MON 11 in the CONV(see FIG. 8) or a maintenance worker informs to the MON CONT 7 that aCONV is added (step B2, SEND ADD-INF), in case the CONV is added. Thenthe MON CONT 7 issues a command to switch the constant output levelcontrol to constant gain control to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81 and another command to raise optical output to the added CONV(step B3, SWITCH CONST-LEVEL-CONT TOCONST-GAIN-CONT, AND RAISE OUTPUT OFADD-CONV).

When the optical output of the added CONV reaches the prescribed value,the MON CONT 7 issues a command to switch the constant gain control backto the constant level control to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81 (step B4, SWITCH BACK TOCONST-LEVEL-CONT). As a result, thetotal optical output of the ELEC-OPT CONV 4, the TX-UNIT OPT-AMP 6 andthe REPOPT-AMP 81 is set to a prescribed value corresponding to thenumber of the operating CONVs, respectively (step B5, OUTPUT OFELEC-OPTCONV, OPT-AMPs 6 and 81 ARE SET TO PRESCRIBED VALUE).

When a CONV is removed from the ELEC-OPT CONV 4 because of, for example,trouble or maintenance, the remove information is sent to the MON CONT 7from a higher rank apparatus or by a maintenance worker (step B6, SENDREMOVE-INF). The MON CONT 7 issues a command to switch the constantoutput level control to the constant gain control to both the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81 and a command to decrease the opticaloutput to the removed CONV (B7, SWITCH CONST-LEVEL-CONTTOCONST-GAIN-CONT AND DECREASE OUTPUT OF REMOVE-CONV). After the opticaloutput of the removed CONV falls to a prescribed value, the MONT CONV 7issues a command to switch the constant gain control back to theconstant output level control to the TX-UNIT OPT-AMP 6 and the REPOPT-AMP 81. Or after a prescribed time passes, the control of theTX-UNIT OPT-AMP 6 and the REP OPT-AMP 81 may switch automatically theconstant gain control back to the constant output level control (stepB8, SWITCH BACK TO CONST-LEVEL-CONT). As a result of step B8, theoperation of the removed CONV is completely stopped and the TX-UNITOPT-AMP 6 and the REP OPT-AMP 81 produce the optical output of aprescribed value corresponding to the number of the operating CONVsexcluding the removed CONV, under the constant output level control(step B9, REMOVE-CONV STOP ANDOPT-AMPs 6 AND 81 PRODUCE OUTPUTCORRESPONDING TO THE NUMBER OF OPERATING CONVs UNDER CONST-LEVEL-CONT).

FIG. 13 is a block diagram of an optical amplifier used to the TX-UNITOPE-AMP 6 and the REP OPT-AMP 81 in the multiplex optical communicationsystem of the third embodiment of the present invention. The opticalamplifier shown in FIG. 13 consists of an optical amplifier unit(OPT-AMP UNIT) (61), an optical output monitoring unit (OUTPUT MON)(62), a gain controlling unit (GAIN CONT) (63), a constant output levelcontrol circuit (CONSTANT OUTPUT CONT) (64), an output level settingunit (LEVEL SET) (65), a control unit (CONT UNIT) (66), a control signalreceiving unit (CONT-SIG RX) (67), a gain monitoring unit (GAIN MON)(68), a constant gain control circuit (CONSTANT GAIN CONT) (69) and aselector (SEL) (70).

In case of the third embodiment, the same CONV as described in referencewith FIG. 8 and the same MON CONT 7 as described in reference with FIG.10 are used. In FIG. 13, the same reference symbol as in FIG. 9designates the same unit or circuit as in FIG. 9, having the samefunction as in FIG. 9. Different from FIG. 9, the optical amplifiershown in FIG. 13 includes the GAIN MON 68, the CONSTANT GAIN CONT 69 andthe SEL 70. The SEL 70 is controlled by the CONT UNIT 66 so that the SEL70 usually selects the CONSTANT OUTPUT CONT 64 for controlling theOPT-AMP UNIT 61 by the GAIN CONT 63.

Upon receiving the optical output control signal from the CONT MON 7,the CONT-SIG RX 67 transfers the optical output control signal to thenext optical amplifier (for example, the REP OPT-AMP 81) and the CONTUNIT 66. When the optical output control signal signifies that the CONVis added or removed, the optical output control signal controls the SEL70 so that the SEL 70 selects the CONSTANT GAIN CONT 69 for connectingthe CONSTANT GAIN CONT 69 with the GAIN CONT 63. As a result, control ofthe optical output from the OPT-AMP UNIT 61 is changed from the constantoutput level control to the constant gain control, so that the OPT-AMPUNIT 61 produces the optical output at an increasing rate of the outputfrom the added CONV or an decreasing rate of the output from the removedCONV. That is, the OPT-AMP UNIT 61 produces the optical output bykeeping the single optical output constant individually.

When a prescribed time determined by the rising time constant of theadded CONV or the falling time constant of the removed CONV is passed,or when the output of the added CONV is in creased to a prescribed largevalue or the output of the removed CONV is decreased to a prescribedsmall value, the optical output control signal sends the aboveinformation to the optical amplifier. Upon receiving the information, inthe optical amplifier, the CONT UNIT 66 controls the SEL 70 so that theSEL 70 selects the CONSTANT OUTPUT-LEVEL CONT 64 for connecting theCONSTANT OUTPUT-LEVEL CONT 64 with the GAIN CONT 63.

FIG. 14 is a block diagram for detailing the optical amplifier shown inFIG. 13. In FIG. 14, the same reference numeral as in FIG. 13 designatesthe same unit as in FIG. 13. In FIG. 14, symbol “ASE” in the OPT-AMPUNIT 61 is an abbreviation of “Amplified Spontaneous Emission” whichwill be described later, the OUTPUT MON 62 consists of an optical branchwhich will be simply called a “brancher” (73) and an optical detector(74), the GAIN CONT 63 consists of an optical combiner (71) and apumping semiconductor laser (72), the GAINMON 68 includes anoptical-to-electrical signal converter (79) using e.g. a photo diode,the LEVEL SET 65 includes a switching circuit (SW) (77), the CONSTANTOUTPUT LEVEL CONT 64 includes an operational amplifier (ope-amp) (76)and the CONSTANT GAIN CONT 69 consists of an averaging circuit (80), aholding circuit (81) and an ope-amp (82), and a transistor (75) is forconnecting the GAINCONT 63 and the SEL 70.

When the CONT UNIT 66 receives the optical output control signalincluding information on starting the added CONV or stopping the removedCONV through the CONT-SIG RX 67, the CONT UNIT 66 controls the SEL 70 sothat the CONSTANT OUTPUT LEVEL CONT 64 is switched to the CONSTANT GAINCONT 69. As a result, a constant gain control loop is formed byconnecting the transistor 75, the semiconductor laser 72, the opticalcombiner 71, the OPT-AMP UNIT 61, the GAIN MON 68 and the CONSTANT GAINCONT 69. By virtue of the constant gain control loop, the OPT-AMP UNIT61 can produce the optical output under the constant gain control.

In this case, the photo diode 79 in the GAIN MON 68 detects the ASEemitted from the Er doped optical fiber in the OPT-AMP UNIT61. Thisdepends on a fact that the intensity of the ASE is directly proportionalto the gain of the Er doped optical fiber. Since the ASE is emitted froma side of the Er doped optical fiber, the ASE can be detected by thephoto diode 79 placed at the side of the Er doped optical fiber. (Thedetails of the above is described in Japanese Patent TOKUKAIHEI4-356984).

A detected signal from the photo diode 79 is sent to the averagingcircuit 80 and averaged thereat, producing an averaged signal. Theaveraged signal is sent to the holding circuit 81 and held thereat untilthe constant gain control is switched to the constant output levelcontrol. The averaged signal held at the holding circuit 81 is sent tothe ope-amp 82. The ope-amp 82 operates so that the detected signal fromthe photo diode 79 becomes always constant to the average signal sentfrom the holding circuit 81. This is performed by controlling theconstant gain control loop. That is, the output from the ope-amp 82controls the transistor 75 through the SEL 70 so that the pumpingsemiconductor laser 72 controls pumping optical power so as to keep thegain of the OPT-AMP UNIT 61 constant.

By virtue of the constant gain control described above, the opticaloutput of the OPT-AMP UNIT 61 is increased or decreased in correspondingto increase or decrease of the optical input of the OPT-AMP UNIT 61.Therefore, the OPT-AMP UNIT 61 can produce the optical output so as tocorrespond to the increase or decrease of the output from the added orremoved CONV respectively. As a result, it becomes possible to add orremove the CONV by keeping the single output of the OPT-AMP UNIT 61constant, which results in decreasing the wave shape distortion of theoptical signal and preventing the decrease of the SNR occurring.

The CONT UNIT 66 makes the SW 77 select one of the reference values rf1,rf2, - - - and rfm in corresponding to the number of the operatingCONVs, and after a prescribed time passed or upon receiving the opticaloutput control signal from the MON CONT 7, the CONT UNIT 66 controls theSEL 70 so that the connecting object of the ope-amp 76 is changed fromthe ope-amp 82 to the ope-amp 76. By virtue of changing the operationalamplifier thus, the pumping semiconductor laser 72 is controlled so thatunder the constant output level control, the OPT-AMP UNIT 61 producesthe optical output in accordance with the reference value set at theLEVEL SET 65.

FIG. 15 is a block diagram of multiplex optical communication system forillustrating a fourth embodiment of the present invention. In FIG. 15,the same reference numeral as in FIG. 3 designates the same unit as inFIG. 3. In FIG. 15, reference numeral 15A indicates a control-signaloptical transmitter (CONT-SIG OPTICAL TX). The CONT-SIG OPTICAL TX 15Atransmits an optical control signal including the optical output controlsignal and having a wavelength different from the wavelengths used inthe CONVs 4-1 to 4-n incase of WDM. The optical control signal is sentto the OPT-SIG COMB 5 by an optical fiber depicted by a thick line inFIG. 15. The optical control signal is combined with other channeloptical signals from the ELEC-OPT CONV 4 and sent to the TX-UNIT OPT-AMPand the REP OPT-AMP 81 through the optical fiber and the OPT-TRANS LINE3. By virtue of using the optical signal, the electric line fortransmitting the optical output control signal becomes unnecessary to belaid.

FIG. 16 is a block diagram for illustrating an optical amplifier used inthe REP OPT-AMP 81 in case of the fourth embodiment. In FIG. 16, thesame reference symbol as in FIG. 9 designates the same unit and has thesame function as in FIG. 9. In FIG. 16, the optical amplifier consistsof an OPT-AMP UNIT (91), an OUTPUT MON (92), a GAIN CONT (93), aCONSTANT OUTPUT CONT (94), a LEVELSET (95), a CONT UNIT (96), a CONT-SIGRX (97), an electro-optical converter (ELEC-OPT CONV) (98), an opticalchannel demultiplexer (OPT-DEMUX) (99) and an optical combiner(OPT-COMB) (100).

The optical amplifier for the fourth embodiment shown in FIG. 16 has thesame constitution as the optical amplifier for the first embodimentshown in FIG. 9 except that the optical amplifier for the fourthembodiment includes the ELEC-OPT CONV 98, the OPT-DEMUX 99 and theOPT-COMB 100. Since the optical output control signal is sent from theMON CONT 7 to the optical amplifier in FIG. 16 through the OPT-TRANSLINE 3 as an optical signal, the optical signal including the opticaloutput control signal is demultiplexed at the OPT-DEMUX 99 and sent tothe CONT UNIT 97. The optical signal is converted to an electricalsignal, which is the optical output control signal, at the CONT UNIT 97.The optical output control signal is again converted to an opticalsignal for transmitting to the REP OPT-AMP 81 in the next OAMP REP EQUIP200. In the optical amplifier for the third embodiment described inreference with FIGS. 12 and 13, it is possible to transmit the opticaloutput control signal to the REP OPT-AMP 81 as an optical signal throughthe OPT-TRANS LINE 3 and convert the optical signal including theoptical output control signal to the electrical signal at the opticalamplifier for the third embodiment.

FIGS. 17A, 17B and 17C are block diagrams for illustrating a multiplexoptical communication system of a fifth embodiment of the presentinvention. Different from the multiplex optical communication system ofthe first embodiment operating under the WDM, the multiplex opticalcommunication system of the fifth embodiment operates under the OTDM. InFIGS. 17A, 17B and 17C, the same reference symbol as in FIGS. 2, 3 and 4designates substantially the same unit as in FIGS. 2, 3 and 4, and thesame reference numeral as in FIG. 3 designates the same object as inFIGS. 2, 3 and 4. In FIGS. 17A, 17B and 17C, the multiplex opticalcommunication system principally consists of two TERM EQUIP (101 and101′), a plurality of OAMP REP EQUIP (102) and the OPT-TRANS LINE 3. TheTERMEQUIP 101 includes a TX-UNIT (1111) which consists of an ELECT-OPTCONV (104), an OPT-SIG COMB (105), a TX-UNIT OPT-AMP (106) and a MONCONT (107) and an RX-UNIT (1102). The ELECT-OPT CONV 104 consists ofCONVs 104-1, 104-2, - - - and 104-n, and each CONV consists of aCONV-CIRCUIT (110), an OPT-OUT MON (111) and OPT-OUT CONT (112). The MONCONT 107 consists of a MON-PROC (113), a CONT-PROC (114) and a CONT-SIGTX (115). The OAMP REP EQUIP 102 includes REP OPT-AMPs (108 and 108′).

The CONVs 104-1 to 104-n transmit optical signals in accordance withelectrical signals or data sent through the ELEC-SIG CHANNELLINES 9, atthe same wavelength, however, different time slot each other. TheOPT-SIG COMB 105 multiplys the optical signals sent from the CONVs 104-1to 104-n in time-divisional and produces optical output as an OTDMsignal. Control of the time slots to the CONVs 104-1 to 104-n isperformed by the CONT-PROC 114 in the MON CONT 107.

The TX-UNIT OPT-AMP 106 and the REP OPT-AMP 108 amplify the OTDM signalunder the time constant control system described in the first embodimentand the control manner change system described in the third embodiment.That is, the optical amplification at the TX-UNIT OPT-AMP 106 and theREP OPT-AMP 108 can be perform by making the rising or falling timeconstant of the TX-UNITOPT-AMP 106 and the REP OPT-AMP 108 equal to therising or falling time constant of the added or removed CONVrespectively, or the optical amplification can be performed by switchingthe control manner as follows: before a CONV is added or removed, theoptical amplification is performed under the constant output levelcontrol as usually performed in the prior art; when a CONV is added andstarts to operate, the optical amplification is performed under theconstant gain control until the optical output of the added CONV reachesa prescribed value, and when a CONV is removed and stops operation, theoptical amplification is performed under the constant gain control untilthe optical output of the removed CONV is decreased to zero value or aprescribed value near to zero; and when the optical output of the addedCONV is raised to the prescribed value, the optical amplification isperformed by the constant output level control, and when the opticaloutput of the removed CONV is decreased to zero value or the prescribedvalue near to zero value, the optical amplification is performed underthe constant output level control. FIG. 18 is illustration of OTDM. Forinstance, four CONVs 104-1, 104-2, 104-3 and 104-4 produce opticaloutput at time slots T1, T2, T3 and T4 as shown by 1-0 signals (a), (b),(c) and (d) in FIG. 18, respectively. Upon receiving four optical outputfrom the CONVs 104-1, 104-2, 104-3 and 104-4, the OPT-SIG COMB 105performs multiplex to the four optical output and produces an OTDM datumas shown by (e) in FIG. 18. Control for making the CONVs 104-1, 104-2,104-3 and 104-4 correspond with the time slots T1, T2, T3 and T4 can beperformed by using an ordinal method used in usual OTDM system.

The present invention can be applied to other multiplex opticalcommunication system employing TDM-OTDM combination system forincreasing multiplex factor and operation efficiency of the multiplexoptical communication system.

What is claimed is:
 1. A system, comprising: a terminal which transmits,to an optical fiber transmission line, an optical signal including aplurality of second optical signals with different wavelengths and asupervisory optical signal, by which condition of the optical signal isnotified, with wavelength different from the second optical signals;and, an optical amplifier including: an amplifying unit which receivesthe optical signal and the supervisory optical signal from the opticalfiber transmission line and amplifies the optical signal; and, acontroller which detects the supervisory optical signal and has a firstmode in which the amplifying unit is controlled to amplify the opticalsignal with an approximately constant gain and a second mode in whichthe amplifying units is controlled to output the amplified opticalsignal with a predetermined level, the controller switchable between thefirst mode and the second mode in response to the supervisory opticalsignal.
 2. A system according to claim 1, wherein a variation of anumber of the second optical signals is notified by the supervisoryoptical signal, and the controller is switchable to the first mode inresponse to the supervisory optical signal.
 3. A system according toclaim 1, wherein steady state of a number of the second optical signalsis notified by the supervisory optical signal, and the controller isswitchable to the second mode in response to the supervisory opticalsignal.
 4. A system according to claim 1, wherein, in the first mode,the amplifying unit is controlled to output each second optical signalwith respective predetermined levels.
 5. A system, comprising: firstmeans for transmitting, to an optical fiber transmission line, anoptical signal including a plurality of second optical signals withdifferent wavelengths and a supervisory optical signal, by whichcondition of the optical signal is notified, with wavelength differentfrom the second optical signals; second means for receiving the opticalsignal and the supervisory optical signal from the optical fibertransmission line and amplifying the optical signal; and, third meansfor detecting the supervisory optical signal and for switching, inresponse to the supervisory optical signal, between a first mode inwhich the second means is controlled to amplify the optical signal withan approximately constant gain and a second mode in which the secondmeans is controlled to output the amplified optical signal with apredetermined level.
 6. A system according to claim 5, wherein avariation of a number of the second optical signals is notified by thesupervisory optical signal, and the third means is switched to the firstmode in response to the supervisory optical signal.
 7. A systemaccording to claim 4, wherein steady state of a number of the secondoptical signals is notified by the supervisory optical signal, and thethird means is switched to the second mode in response to thesupervisory optical signal.
 8. A system according to claim 5, wherein,in the first mode, the second means is controlled to output each secondoptical signal with respective predetermined levels.
 9. An opticaltransmission system, comprising: a terminal station which transmits, toan optical fiber transmission line, an optical signal including aplurality of second optical signals with different wavelengths and asupervisory optical signal, by which condition of the optical signal isnotified, with wavelength different from the second optical signals; anoptical amplifier which, optically coupled to the optical fibertransmission line, receives the optical signal and amplifies the opticalsignal; and, a controller, receiving the supervisory optical signal fromthe optical fiber transmission line, operatively coupled to the opticalamplifier and switched, in response to the supervisory optical signal,between a first mode in which the optical amplifier is controlled toamplify the optical signal with an approximately constant gain and asecond mode in which the optical amplifier is controlled to output theamplified optical signal with a predetermined level.
 10. An opticaltransmission system according to claim 9, wherein a variation of anumber of the second optical signals is notified by the supervisoryoptical signal, and the controller is switched to the first mode inresponse to the supervisory optical signal.
 11. An optical transmissionsystem according to claim 9, wherein steady state of a number of thesecond optical signals is notified by the supervisory optical signal,and the controller is switched to the second mode in response to thesupervisory optical signal.
 12. An optical transmission system accordingto claim 9, wherein, in the first mode, the optical amplifier iscontrolled to output each second optical signal with respectivepredetermined levels.
 13. A system, comprising: a terminal whichtransmits, to an optical fiber transmission line, an optical signalincluding a plurality of second optical signals with differentwavelengths and a supervisory optical signal, by which condition of theoptical signal is notified, with wavelength different from the secondoptical signals; and, an optical amplifier including: an amplifying unitwhich receives the optical signal and the supervisory optical signalfrom the optical fiber transmission line and amplifies the opticalsignal; and, a controller which detects the supervisory optical signaland has a first mode in which the amplifying unit is controlled tooutput the amplified second optical signals with respectivepredetermined levels and a second mode in which the amplifying units iscontrolled to output the amplified optical signal with a predeterminedlevel, the controller switchable between the first mode and the secondmode in response to the supervisory optical signal.
 14. A systemaccording to claim 13, wherein a variation of a number of the secondoptical signals is notified by the supervisory optical signal, and thecontroller is switchable to the first mode in response to thesupervisory optical signal.
 15. A system according to claim 13, whereinsteady state of a number of the second optical signals is notified bythe supervisory optical signal, and the controller is switchable to thesecond mode in response to the supervisory optical signal.
 16. A system,comprising: first means for transmitting, to an optical fibertransmission line, an optical signal including a plurality of secondoptical signals with different wavelengths and a supervisory opticalsignal, by which condition of the optical signal is notified, withwavelength different from the second optical signals; second means forreceiving the optical signal and the supervisory optical signal from theoptical fiber transmission line and amplifying the optical signal; and,third means for detecting the supervisory optical signal and forswitching, in response to the supervisory optical signal, between afirst mode in which the second means is controlled to output theamplified second optical signals with respective predetermined levelsand a second mode in which the second means is controlled to output theamplified optical signal with a predetermined level.
 17. A systemaccording to claim 16, wherein a variation of a number of the secondoptical signals is notified by the supervisory optical signal, and thethird means is switched to the first mode in response to the supervisoryoptical signal.
 18. A system according to claim 16, wherein steady stateof a number of the second optical signals is notified by the supervisoryoptical signal, and the third means is switched to the second mode inresponse to the supervisory optical signal.
 19. An optical transmissionsystem, comprising: a terminal station which transmits, to an opticalfiber transmission line, an optical signal including a plurality ofsecond optical signals with different wavelengths and a supervisoryoptical signal, by which condition of the optical signal is notified,with wavelength different from the second optical signals; an opticalamplifier which, optically coupled to the optical fiber transmissionline, receives the optical signal and amplifies the optical signal; and,a controller, receiving the supervisory optical signal from the opticalfiber transmission line, operatively coupled to the optical amplifierand switched, in response to the supervisory optical signal, between afirst mode in which the optical amplifier is controlled to output theamplified second optical signals with respective predetermined levelsand a second mode in which the optical amplifier is controlled to outputthe amplified optical signal with a predetermined level.
 20. An opticaltransmission system according to claim 19, wherein a variation of anumber of the second optical signals is notified by the supervisoryoptical signal, and the controller is switched to the first mode inresponse to the supervisory optical signal.
 21. An optical transmissionsystem according to claim 19, wherein steady state of a number of thesecond optical signals is notified by the supervisory optical signal,and the controller is switched to the second mode in response to thesupervisory optical signal.